Technical Abstract:
While livestock experience many stressors throughout the production cycle, one of the most commonly experienced, and most difficult to control, is stress caused by fluctuations in environmental temperatures (ET) that expand beyond the thermal neutral zone for an animal. In swine, cold stress has long been recognized as a major cause of neonatal morbidity and mortality, and is considered to be a major contributing factor associated with high percentages of neonatal losses (Le Dividich and Noblet, 1981). Previous studies have demonstrated that maintaining neonatal pigs in cold ET for the first 15 days of life results in decreased growth and increased number of deaths (Heath, 1989). Additionally, it has been suggested that cold ET may interfere with the neonatal pig's ability to clear pulmonary bacterial infections (Curtis et al., 1976). A possible explanation for the increased morbidity and mortality associated with cold stress in neonatal pigs may be related to the inability of the neonatal pig to generate a febrile response. Previously, we have demonstrated that the acute phase immune response, including the generation of fever, following exposure to a provocative immune challenge is substantially altered in neonatal pigs maintained in a cold environment (18 degrees C) as compared to those maintained in a thermal neutral environment (34 degrees C) (Carroll et al., 2001). In fact, neonatal pigs that were maintained in a cold environment and exposed to a bacterial endotoxin (lipopolysaccharide; LPS) experienced a period of hypothermia which in and of itself could prove detrimental in a production environment. These data demonstrate a strong linkage between ET and the ability of the neonatal pig to mount an adequate immunological response, as the primary function of a febrile response is to assist in the removal of invading pathogens. In cattle, we have previously reported differences in the acute phase immune response in two diverse breeds of Bos taurus (Angus and Romosinuano) cattle when maintained under thermal neutral (TN) conditions and exposed to LPS (Carroll et al., 2011). In a more recent study evaluating these two breeds of cattle, Angus heifers displayed greater rectal temperature than Romosinuano heifers when housed at either TN (cycling from 18.5 to 23.5 degrees C) or heat stress (HS; cycling from 18.5 to 38 degrees C) temperatures and produced a greater febrile response to LPS administration. Sickness behavior scores in response to LPS administration were similar between breeds when housed in at TN. However, when housed in a HS environment, Romosinuano heifers displayed greater sickness behavior scores compared to Angus heifers following LPS administration. Environmental temperature also played a significant role in the release of pro-inflammatory cytokines following LPS administration. For example, heifers held in a HS environment produced greater concentrations of interleukin-6 after LPS administration than heifers housed in at TN. These data clearly demonstrate that even intermittent periods of HS similar to that experienced in production environments (e.g. tropical climates) can have significant effects on the stress and innate immune responses of cattle. Understanding the impact of thermal stress on the immunological responses of livestock is critical to developing and implementing alternative management practices that would improve the overall health and well-being of animal in production systems.